Apple’s iPhone on Thursday took first place for the fourth year running in J.D. Power and Associates’ smartphone customer satisfaction rankings.

Americans are also keeping their mobile phones longer than ever, the market research firm said, and paying monthly bills 13 per cent higher than three years ago.

Apple scored 800 out of a possible 1,000 points in J.D. Power’s customer satisfaction survey, leading the two closest rival makers Motorola, with 791, and HTC, with 781. Apple’s margin over the second and third place makers, however, was narrower than last year, when the iPhone beat LG and Samsung smartphones by 11 and 19 points, respectively.

The industry average for smartphone satisfaction was 754.

Although J.D. Power did not single out any specific model or models made by Motorola or HTC, both companies have bet heavily on Android. Motorola, for instance, markets the Droid X and Droid 2, while HTC sells the Droid Incredible and Evo 4G. All rely on Google’s mobile operating system.

J.D. Power measured customer satisfaction by weighting several question categories, giving prominence to ease of operation and the smartphone’s operating system — each worth 26 per cent of the final score — and discounting battery functionality (just 8 per cent of the total).

The battery category was the only one of the five that Apple did not ace.

J.D. Power’s nod wasn’t’ the first for Apple’s iPhone this year.

Last July, surveys conducted by market research company ChangeWave said that 93 per cent of iPhone 4 owners checked “very satisfied” or “somewhat satisfied” when asked how they felt about their phones. Although that total was down from the 99 per cent who said the same the year before about the then-new iPhone 3GS, ChangeWave’s director of research said Apple should be happy with the results, all things considered.

“I think it’s quite an accomplishment that this satisfaction [rating] stayed up that high after all this,” Carton said at the time.

The “this” Carton referred to was the brouhaha over the iPhone 4’s dropping calls when it was held in certain ways. Apple CEO Steve Jobs derided the public relations mess as “Antennagate” during a hastily called news conference on July 16, during which he defended the iPhone 4, said competitors’ phones suffered the same problem, and promised every iPhone 4 owner a free case to quell the complaints.

One of the few dissenting voices this year has been Consumer Reports magazine, which has refused to recommend the iPhone 4 even though the smartphone scored highest in its evaluation.

Consumer Reports first called out Apple over the antenna issue in July, and this month repeated its “not recommended” position after Apple announced it would halt the free case program at the end of this month.

J.D. Power’s survey also noted that U.S. mobile phone owners were holding on to their handsets for an average of 20.5 months, the longest stretch since the company started tracking cell phones in 1999.

Kirk Parsons, J.D. Power’s senior director of wireless services, had a theory to explain the trend. “One possible reason is that more customers are delaying an upgrade purchase due to the general economic downturn,” he said in a statement. “Consumers are really watching their wallets.”

Another contributing factor may be the higher monthly mobile bills people are paying. The average monthly wireless bill in 2010 was $78, a 13 per cent jump from 2007’s $69.